Cementing spacers for improved well cementation

ABSTRACT

A method of cementing a well using a cementing spacer. The method includes pumping a drilling fluid into a well. A cementing spacer is then pumped into the well to serve as a buffer between the drilling fluid and a cement. The cementing spacer includes substantially unviscosified water and a weighting agent. Cement is then pumped into the well to displace the cementing spacer and the drilling fluid to complete the cementing of the well.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/031,083 filed on Feb. 26, 1998.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to cementing spacers for use incementing wellbores. More specifically, the invention relates to the useof Stokes Law cementing spacers when cementing wells.

[0004] 2. Background Art

[0005] When drilling an oil or gas well, drilling fluid having aprescribed density is used during the drilling operation for severalpurposes including, for example, balancing a formation fluid pressure(which generally increases as the depth of a well increases) present ingeologic formations that are penetrated by the wellbore. The drillingfluid, or “drilling mud,” is typically pumped down a drillstring,through a drill bit, and is returned to the surface though an annulusformed between the drillstring and a wall of the wellbore. This processis known as “circulation” of the drilling fluid.

[0006] If the density of the drilling fluid is excessive, thehydrostatic pressure exerted by the drilling fluid on the formations canresult in fractured formations and a resultant loss of drilling fluidinto the “broken down” formations. Loss of drilling fluid into theformation typically results in “lost circulation” (e.g., the loss of areturn fluid communication path to the surface through, for example, thewellbore annulus) and eventually a pressure underbalance with respect toformation fluid pressure. Lost circulation can result in uncontrolleddischarge or “blowouts” of pressurized formation fluids to the surfacebecause pressure control of the well has been lost. For example, whendrilling fluid is lost into the formation, the wellbore pressure dropsand permits higher pressure formation fluids to flow into the wellborein the form of a “kick.” The kick may propagate to the surface andresult in a blowout that can damage rig equipment and injure or kill rigpersonnel.

[0007] These conditions may generally be avoided by appropriateselection of the density of the drilling fluid used to drill the well.The density of the drilling fluid is usually controlled by the additionof “weighting agents” in the form of, for example, particulate solids ofheavy earth materials, such as barite. The weighting agents are added tothe drilling fluid in a known ratio with respect to the fluid volume inthe wellbore to produce a carefully regulated drilling fluid with aknown density.

[0008] During the drilling process, it is often necessary toperiodically lower steel casing or well liners into the wellbore to linethe walls thereof in order to maintain stability of the wellbore.Moreover, the casing may be required to protect shallower formationsfrom the high wellbore pressures required to maintain fluid pressurebalance or overbalance with respect to formations near the bottom of thewellbore. The casing, which is typically steel, must fit inside thewellbore diameter.

[0009] After the casing is placed in the wellbore, an external casingannulus is formed between an outer surface of the casing and the wall ofthe wellbore. In order to prevent fluid communication along the externalcasing annulus, oil well cement is typically pumped into the externalcasing annulus. Cementation of the casing in the wellbore is importantbecause undesirable fluid communication between the bottom of thewellbore and the surface through the external casing annulus can resultin formation fluid leakage to the surface or to other subsurfaceformations, and can result in other types of well damage resulting in aloss of production potential. The oil well cement is placed in theexternal casing annulus by pumping a substantially fluid cement slurrydown the casing, out of the bottom of the casing, and up into theexternal casing annulus.

[0010] During the cementing process, the cement slurry must completelydisplace the drilling fluid from the external casing annulus becausedrilling fluid that is not displaced may provide a path for the flow offormation fluids up the external casing annulus after the cement hasset. Moreover, slurries of oil well cement are often not chemicallycompatible with common drilling fluids. For example, if the cementslurry comes into direct contact with the drilling fluid during thedisplacement process, the cement slurry and the drilling fluid may mixtogether and form a viscous material. When the cement slurry is pumpedinto the external casing annulus, the cement slurry may bypass theviscous material, thereby leaving channels of viscous material that donot set up to form a solid, impermeable cement barrier to formationfluids. Accordingly, a cement “spacer” fluid is often pumped into thewellbore between the drilling fluid and the cement slurry to improve thedisplacement of the drilling fluid and to prevent direct contact andmixing of the drilling fluid and the cement slurry.

[0011] Again, however, the density of such a cement spacer cannot exceedcertain limits or the lost circulation condition will be encountered,and it cannot fall below other certain limits or an underbalancedcondition will occur. Thus it is necessary to be able to control thedensity of the spacer fluid used in cementing operations in a mannersimilar to that used to control the density of drilling fluid duringdrilling operations.

[0012] Prior art spacers are generally made by mixing a suitable liquidbase fluid with a viscosifier which may be, for example, a solublepolymer or bentonite clay and a weighting agent including, for example,solid particles of barite or calcium carbonate. The weighting agent mayalso include low-density particles such as hollow glass or ceramicspheres or foamed nitrogen. The most common spacer base fluid is plainwater. “Plain water” includes, for example, any source of chemicallysuitable water that is readily available for such applications,including fresh water, seawater, saltwater, and brine. Alternatively, asuitable organic solvent may be used as the spacer base fluid. Organicsolvents are often advantageous for use in displacing oil based drillingfluids. When used in this manner, organic solvents may also includeviscosifiers and weighting agents.

[0013] In prior art cement spacer fluids, the viscosifier is used tosupport the particles of weighting agent so as to prevent settling ofthe weighting agent during the pumping operation. A performanceobjective of the viscosifier is often to develop “gel strength” understatic conditions to aid in the support of the weighting agentparticles. Particle settling in cement spacer fluids is usuallyevaluated in laboratories with settling tests, similar to the API freewater test used for cement slurries. In the test, the volume of freewater, which accumulates on the top of the spacer under specifiedconditions, is determined. A common practice is to require that the freewater be below some maximum volume.

[0014] Therefore, it is desirable to have a cementing spacer that isdesigned to displace drilling fluid in a wellbore and serve as a bufferbetween the drilling fluid and a cement slurry used to cement, forexample, casing in a wellbore. Moreover, it is desirable to have acementing spacer that can be readily formed at a well site.

SUMMARY OF INVENTION

[0015] A method of cementing a well using a cementing spacer. The methodcomprises pumping a drilling fluid into a well and pumping a cementingspacer into the well to displace the drilling fluid, wherein thecementing spacer comprises substantially unviscosified water and aweighting agent. Cement is then pumped into the well to displace thecementing spacer and the drilling fluid to complete the cementing of thewell.

[0016] Other aspects and advantages of the invention will be apparentfrom the following description and the appended claims.

DETAILED DESCRIPTION

[0017] Embodiments of the invention have been developed from a study ofparticle settling calculations based on the Stokes-Einstein equation.The particle settling calculations show that the sedimentation rate, orparticle settling velocity, of particles of a weighting agent in a basefluid is relatively slow when compared to the depth of a typical well.For example, a total sedimentation distance of about 40 feet in a 4 hourperiod was calculated for particles of a calcium carbonate weightingagent in a water base fluid.

[0018] Accordingly, if a controlled density cementing spacer comprisingcalcium carbonate and water (and substantially no viscosifier) is pumpedinto an external casing annulus in a substantially vertical wellbore,particles of calcium carbonate in the cementing spacer typically willsettle no more than about 40 feet by the time the cement has set. Thisdegree of settling will not cause any operational problems with respectto cementing the well.

[0019] In an embodiment of the invention, the cementing spacer comprisesa weighting agent (such as, for example, calcium carbonate, barite,ferrite, hematite, etc.) and water. Note that as previously disclosed,“water” may include fresh water, salt water, seawater, brine, or anyother chemically suitable source of water that will not adversely reactwith drilling mud or the cement in the wellbore. The cementing spacersdescribed above are typically referred to as “Stokes Law” mixtures. Theresulting cementing spacers have numerous advantages discussed belowwhen compared to prior art spacers that use viscosifiers to support theweighting agent.

[0020] One form of the Stokes-Einstein equation is shown below asEquation 1: $\begin{matrix}{V = \frac{2{{gr}^{2}\left( {d_{p} - d_{f}} \right)}}{9v}} & (1)\end{matrix}$

[0021] wherein V is a particle settling velocity (cm/sec), g isgravitational acceleration (980 cm/sec²), r is a particle radius (cm),d_(p) is a particle density (g/cm³), d_(f) is a fluid density (g/cm³),and v is a fluid viscosity (poise).

[0022] Numerical solutions of Equation 1 have been determined fordifferent types of particulate weighting agents (such as, for example,calcium carbonate, barite, ferrite, hematite, etc.) and differentparticle diameters. Moreover, the solutions have been determined usingwater or organic solvents. The results show that, compared to a depth ofa well (e.g., the overall height of a cement annulus from a casingbottom to a well head) and to a length of a typical casing string, thesedimentation velocity (or sedimentation rate) of particles of theweighting agent is substantially slow.

[0023] Therefore, based on calculations performed using Equation 1, ithas been determined that cementing spacers comprising water and aweighting agent have a substantially slow particle settling velocity sothat they may be pumped into a well using typical rig operatingtechniques and do not require the addition of a viscosifier (such asbentonite or viscosifying polymers) to impede particle settlement orotherwise affect the rheology of the cementing spacer. A small amount ofviscosifier may be present in the cementing spacer as long as the amountdoes not substantially affect the rheology of the cementing spacer(e.g., as long as the amount of viscosifier does not substantiallyaffect the settling properties of the weighting agent). These cementingspacers do not adversely affect cement slurries used to cement wells,and avoidance of the use of viscosifiers may have several advantages,including:

[0024] Cementing spacers are less expensive because they comprise fewercomponents.

[0025] Cementing spacers have predictable properties resulting in lesspilot testing and quality control requirements.

[0026] The reduction or absence of gel strength development, combinedwith the settling motion of the cementing spacer particles, maintainshydrostatic pressure on the cement slurry as it sets and therebyprovides a better seal through producing zones.

[0027] Cement bond well logs are improved.

[0028] Cementing spacers have a substantially Newtonian rheology andexperience turbulent flow at lower pumping rates and thereby improve thedisplacement of drilling fluid (in the external casing annulus) by thecement slurry.

[0029] Less mixing occurs at the interface between the turbulent flowcementing spacer and the drilling fluid, which also improves thedisplacement of the drilling fluid.

[0030] While the cementing spacers comprise substantially unviscosifiedwater and a weighting agent, other non-viscosifying additives may beused as well. For example, friction reducing additives may be used withthe invention. Friction reducing additives may also serve to eitherminimize or enhance solid packing of particles of the weighting agent.

[0031] Moreover, during extended settling conditions (e.g., settlingconditions that continue for some time after the cement has set),particles of the weighting agent (which may comprise, for example,barite) in the cementing spacer settle and may form a “plug” (e.g., a“barite plug”) proximate the top of a cement column. The plug forms anadditional seal and further prevents fluid transmission from the bottomof the wellbore to the surface. The additional sealing properties of theplug may be useful, for example, in meeting regulatory requirementsassociated with, for example, external casing pressure and/ormicroannular gas leakage (a condition that results from the formation ofa small microannulus or gap between the set cement and the casing and/orthe formation which may allow slow leakage of gas to the surface).

[0032] Stokes Law calculations also apply to the “particle rise” ofparticles of low density weighting agents that may be added to thecementing spacer. For example, the use of hollow glass or ceramicspheres, foamed nitrogen, etc., to lower or reduce the density of thecementing spacer may also be used in embodiments of the invention.Further, the cementing spacers may be used to recover expensive oilbased drilling fluids from wells for future reuse.

[0033] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method of cementing a well using a cementingspacer, the method comprising: pumping a drilling fluid into a well;pumping a cementing spacer into the well to displace the drilling fluid,the cementing spacer comprising substantially unviscosified water and aweighting agent; and pumping cement into the well to displace thecementing spacer and the drilling fluid and to thereby complete thecementing of the well.
 2. The method of claim 1 , wherein the weightingagent comprises barite.
 3. The method of claim 1 , wherein the weightingagent comprises calcium carbonate.
 4. The method of claim 1 , whereinthe cementing spacer further comprises a friction reducing additive. 5.The method of claim 1 , wherein the cementing spacer further comprises alow density weighting agent.
 6. The method of claim 5 , wherein the lowdensity weighting agent comprises a selected volume of hollow glassspheres.
 7. The method of claim 5 , wherein the low density weightingagent comprises a selected volume of ceramic spheres.
 8. The method ofclaim 5 , wherein the low density weighting agent comprises a selectedvolume of foamed nitrogen.